The flow rate of a fluid in a pipe may be measured by means of an ultrasonic flowmeter, according to the method for measuring the difference of transit time durations. This means is nowadays widespread thanks to progress in electronics which allow measuring time intervals with a high resolution.
A known type of ultrasonic flowmeter, presented in particular by the document US-A1-20140345390, includes a fluid passage cylinder comprising at each end an inlet or an outlet for the fluid flowing into a direction perpendicular to this cylinder.
Each end of the fluid passage cylinder has a closure face perpendicular to this cylinder, receiving, at the outside on one side an ultrasonic emitter, and on the other side a receiver receiving the waves emitted by the emitter.
The cylinder further includes a tube fitted inside, presenting an axial perforation forming a channel receiving the flow rate of the fluid as well as the ultrasonic wave sent by the emitter. The tube made of an ultrasound-absorbing material, such as a polymer, achieves an attenuation of the propagation of the sound waves scattering on the sides of the channel, in order to avoid parasitic propagations of the signal resulting in a noise on the receiver in addition to the relevant signal transmitted only by the fluid, which is expected by the receiver.
To the propagation velocity of the sound signal in the fluid, is added the fluid velocity resulting in a small variation of the time interval between the emission by the emitter and the reception by the receiver, which is measured. Knowing moreover the characteristics of the fluid and the geometry of this fluid passage, a fluid velocity as well as the volume flow rate are deduced.
Nonetheless, this type of flowmeters may raise problems because the tube attenuating the propagation of sound waves still lets parasitic waves pass which deliver a measurement noise on the receiver.
Particularly, a complementary measuring method using this type of flowmeter consists in measuring the propagation times of an ultrasonic wave between an emitter and a receiver, called transducers, in a flowing of the fluid, when this wave is emitted co-currently in the direction of the flow, and counter-currently in the reverse direction.
For this purpose, the upstream transducer is energized which emits a co-current wave, which is received by the downstream transducer. Then, the downstream transducer is energized which emits a counter-current wave, which is received by the upstream transducer. Thus, each transducer alternately switches from an emission state to a reception state.
Since the distance between the two transducers is known, the difference between the two transit times of the wave allows determining the flowing velocity of the fluid according to the physical characteristics of the latter, mainly the pressure and the temperature. The geometry of the conduit allows afterwards calculating the volume flow rate of the flowing.
Particularly with this type of measuring method, the measurement noise coming from the parasitic waves transmitted by the solid body of the measuring channel, may be troublesome by disturbing the accuracy of the measurement of the transit times of the wave in the fluid.